1. Field of the Invention
The present invention relates to a magnetoresistant transduction device for reading low density coded data. It is of particular use in franking or mailing machines in order to detect and check as to whether the data printed by these machines on the envelopes to be stamped, corresponds to the printing instructions.
2. Description of the Prior Art
In large undertakings, large offices and more commonly in all premises at which an economic and/or social human activity is carried on, and in which the mail dealt with and dispatched is numerically very substantial, it is impractical to frank the latter in conventional manner, that is to say manually by means of stamps. Instead of these, the value of the postage is imprinted directly on the envelope. Thus, for a postage value of twenty cents, for example, the FIG. 20 is imprinted on the envelope to be dispatched, instead of affixing a twenty cent stamp on this letter.
The operation which consists in imprinting the mail franking values is performed by means of "franking machines", which are now well known and in widespread use. For example, such machines are produced and commercially available from the French company, SECAP, located at 21, Quai Legallot, Boulogne Billancourt, France.
In these machines, the printed postage values may comprise up to four digits, two of these being situated in front of the decimal point and two behind the latter.
The principle of franking machine is comparatively simple. These machines comprise a revolving shaft in which are formed a plurality of grooves of which the axis of symmetry is parallel to the shaft generatrix, i.e., axis. These grooves have installed in them rods having a length greater than that of the shaft. There are as many rods as there are digits in the postage value, generally four.
At one of its extremities, each rod is integral with a first wheel bearing which has a number of projections at its periphery each being able to come into contact with this extremity. This wheel is itself integral with a second wheel which, at its periphery, bears ten printing symbols which corresponding to the numerals from 0 to 9, each symbol being allocated to a given circumferential position of the first wheel.
Each of the four rods may slide within the groove in which it is situated. To this end, it comprises a rack at its other extremity opposite to that which is in contact with the wheel bearing the projections. Each of the racks is associated with and controlled by a code barrel wheel. In order to print one of the four numerals of a postage figure, the corresponding coding wheel is caused to turn until the displacement of the associated rod causes rotation of the wheel bearing the printing symbols through an angle such that the symbol corresponding to the numeral it is intended to print comes into contact with the letter which is to be franked and thereby prints the required numeral.
It is, however, of importance to check on whether the number actually printed corresponds to the number for which a printing order has been issued by means of the coding wheel.
To allow this check to be carried out, each rod is provided with a set of notches delineating a group of gaps and teeth. The four rods are preferably identical, that is to say, comprise the same combination of gaps and teeth. The set of four rods is associated with a transduction device which is stationary.
The rod occupies a different position within the groove, each time a given numeral is printed. Since the transduction device is stationary, it is apparent that when one of the four rods is placed in front of the transduction device, the transduction device is confronted by the position occupied by the rod.
In other words, depending on the position occupied by the rod within the groove, the transduction device is confronted with ten different possible combinations of gaps and teeth. The transduction device preferably supplies a set of five signals for each combination of gaps and teeth. Ten different sets of five signals which consequently constitute a mode, clearly correspond to the ten different combinations of gaps and teeth.
A signal of a first kind (for example, a positive pulse) corresponds to a gap. A signal of a second kind (for example, a zero value pulse) corresponds to a tooth. Consequently, it is apparent that the transduction device supplies a set of five binary signals. As a rule, this set of five binary signals is decoded by an appropriate electronic circuit enabling the franking machine operator to check whether the figure printed by the machine is actually the figure which had been instructed to be printed.
In current practice, the transduction device utilized in franking machines are transduction devices comprising magnetoresistances.
It will be recalled that a magnetoresistance is an element formed of a magnetic material of which the electrical resistance R varies as a function of the magnetic field to which it is exposed, these magnetoresistances being situated on a substrate of electrically insulating material.
Let us consider a measuring magnetoresistance R connected to the terminals of a current generator which delivers a current having the intensity I flowing in the direction of the length of the magnetoresistance. When this magnetoresistance is exposed to a magnetic field H, the latter causes a change .DELTA.R of its resistance, leading to a variation .DELTA.V=I.times..DELTA.R at its terminals, which gives .DELTA.V/V=.DELTA.R/R in which .DELTA.R/R is referred to as the "magnetoresistance factor". It is thus apparent that the voltage variation picked up at the terminals of the magnetoresistance is greater the higher the resistance R.
The electrical signal derived across the terminals of a magnetoresistance is a function only of the value of the field H to which it is exposed.
It will be recalled that the ratio between B and H, that is between magnetic induction and the magnetic field itself when B and H are close to zero, and this on the first magnetization curve, is referred to as "initial magnetic permeability of a magnetic material". (It will be recalled that the first magnetization curve is the curve plotting the variation of B as a function of H when the magnetoresistance is exposed to a magnetic magnetizing field, and starts from an initial magnetic state of the material defined by B and H being close to zero). In other words, the initial magnetic permeability of the magnetic material is equal to the slope of the first magnetization curve close to the point B=0 and H=0. On the other hand, it will be recalled that a magnetically anisotropic material arranged in a plane, meaning that its thickness is much smaller than its length and equally than its width, has two preferential generally mutually perpendicular directions of magnetization within itself. One of these is referred to as the "direction of easy magnetization", whereas the other is referred to as the "direction of difficult magnetization". The initial permeability of the material in the direction of difficult magnetization is much greater than the initial permeability of the material in the direction of easy magnetization.
In current practice, the transduction devices comprising magnetoresistances which are utilized in franking machines, comprises five magnetoresistances of which the length is much greater than the width and which are all aligned, meaning that they have the same direction and are all arranged on one and the same straight line. These are so-called "thick-layer" magnetoresistances, that is to say, which have dimensions of the following order of magnitude: the length L is of the order of 10 millimeters, whereas the width l and the thickness e are of the order of 2 millimeters. The five aligned magnetoresistances are situated on a substrate of plastic material within small openings formed in this substrate. The substrate itself is arranged on a magnetization device, for example, formed by a permanent magnet, most often having a parallelepipedal form.
A transduction device of this nature operates in the following manner: (only one magnetoresistance will be considered since each of these operates in the same manner). When the magnetoresitance is positioned before a tooth of the rod situated in front of it, the magnetic field to which it is exposed is equal to H.sub.1. Let as assume that the latter is positive. The resistance of the magnetoresistance is then R.sub.1. When this same magnetoresistance is positioned in alignment with a gap, it is exposed to a field H.sub.2 which is still positive but smaller than H.sub.1. It is apparent that upon passing from a gap to a tooth, there is a negative magnetic field variation (H=(H.sub.2 -H.sub.1) causing a positive resistance variation of the magnetoresistance, namely .DELTA.R, the resistance of the magnetoresistance then being R.sub.2 =R.sub.1 +.DELTA.R. A voltage change .DELTA.V=.DELTA.R.times.I consequently occurs across the terminals of the magnetoresistance. It is apparent that upon passing from the tooth to the gap, the magnetic field change H.sub.1 -H.sub.2 is caused essentially by the deformation of the magnetic field lines generated by the magnetization device. The permeability of the magnetic medium forming the tooth obviously differs from the permeability of the air filling the gap. In other words, upon passing from a tooth to a gap, the magnetoresistance supplies a positive voltage pulse .DELTA.V. It is clear that, conversely, upon passing from a gap to a tooth, the magnetoresistance supplies a negative voltage pulse. The pulses are easy to detect by means of appropriate conventional electronic circuits, whether the magnetoresistance is positioned in alignment with a gap or with a tooth.
Magnetoresistant transduction devices of this nature are comparatively costly since they require several successive production processes corresponding to the mounting of each of the magnetoresistances. Furthermore, a very precise mechanical fitting operation is required to position each of the magnetoresistances within the opening in which it is to be located.
Consequently, it will henceforth be preferable to make use of magnetoresistances formed by thin layers of very small thickness (from a few hundered .ANG.ngstrom to a few microns). Nevertheless, it is demonstrable that the signal supplied by each magnetoresistance in response to the magnetic field to which it is exposed, diminishes in appreciable manner as a function of the distance separating the magnetoresistance from the rod of said shaft, in front of which it is situated. It is, therefore, necessary that this distance should be comparatively small. Under these circumstances, the magnetization device of the magnetoresistant transduction device being common to the set of magnetoresitances, and the distance between the magnetoresistances and the rods bearing the set of gaps and teeth being comparatively small, interactions then intervene between the different magnetic fields to which these magnetoresistances are exposed. In other words, the different magnetic fields to which the magnetoresistances are exposed are not independent and there is a diaphony of the signal supplied by each of the magnetoresistances.